Abrasive compacts are used extensively in cutting, milling, grinding, drilling and other abrasive operations. An abrasive particle compact is a polycrystalline mass of abrasive particles, such as diamond and/or cubic boron nitride (CBN), bonded together to form an integral, tough, high-strength mass. Such components can be bonded together in a particle-to-particle self-bonded relationship, by means of a bonding medium disposed between the particles, or by combinations thereof. The abrasive particle content of the abrasive compact is high and there is an extensive amount of direct particle-to-particle bonding. Abrasive compacts are made under elevated or high pressure and temperature (HP/HT) conditions at which the particles, diamond or CBN, are crystallographically stable. For example, see U.S. Pat. Nos. 3,136,615, 3,141,746, and 3,233,988.
A supported abrasive particle compact, herein termed a composite compact, is an abrasive particle compact, which is bonded to a substrate material, such as cemented tungsten carbide.
Abrasive compacts tend to be brittle and, in use, they frequently are supported by being bonded to a cemented carbide substrate. Such supported abrasive compacts are known in the art as composite abrasive compacts. Compacts of this type are described, for example, in U.S. Pat. Nos. 3,743,489, 3,745,623, and 3,767,371. The bond to the support can be formed either during or subsequent to the formation of the abrasive particle compact. Composite abrasive compacts may be used as such in the working surface of an abrasive tool.
Composite compacts have found special utility as cutting elements in drill bits. Drill bits for use in rock drilling, machining of wear resistant materials, and other operations which require high abrasion resistance or wear resistance generally consist of a plurality of polycrystalline abrasive cutting elements fixed in a holder. U.S. Pat. No. 4,109,737 describes drill bits with a tungsten carbide stud (substrate) having a polycrystalline diamond compact on the outer surface of the cutting element. A plurality of these cutting elements then are mounted generally by interference fit into recesses into the crown of a drill bit, such as a rotary drill bit. These drill bits generally have means for providing water-cooling or other cooling fluids to the interface between the drill crown and the substance being drilled during drilling operations. Generally, the cutting element comprises an elongated pin of a metal carbide (stud) which may be either sintered or cemented carbide (such as tungsten carbide) with an abrasive particle compact (e.g., polycrystalline diamond) at one end of the pin for form a composite compact.
Fabrication of the composite compact typically is achieved by placing a cemented carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and compressed under HP/HT conditions. A composite compact formed in the above-described manner may be subject to a number of shortcomings. For example, the coefficients of thermal expansion and elastic constants of cemented carbide and diamond are close, but not exactly the same. Thus, during heating or cooling of the polycrystalline diamond compact (PDC), thermally induced stresses occur at the interface between the diamond layer and the cemented carbide substrate, the magnitude of these stresses being dependent, for example, on the disparity in thermal expansion coefficients and elastic constants.
Another potential shortcoming, which should be considered, relates to the creation of internal stresses within the diamond layer, which can result in a fracturing of that layer. Such stresses also result from the presence of the cemented carbide substrate and are distributed according to the size, geometry, and physical properties of the cemented carbide substrate and the polycrystalline diamond layer. In some applications, the tools are subject to delamination failures caused by thermally induced axial residual stresses on the outer diameter of the superabrasive layer. The stresses reduce the effectiveness of the tools and limit the applications in which they can be used.
Recently, various PDC structures have been proposed in which the diamond/carbide interface contains a number of non-planar features designed to increase the mechanical bond and reduce thermally induced residual stresses. For example, U.S. Pat. No. 5,351,772 presents various interface designs containing radial raised lands on the substrate. However, high tensile residual stresses still exist at the diamond surface and near the interface in those designs. U.S. Pat. No. 5,484,330 suggests a saw tooth shaped cross-sectional profile and U.S. Pat. No. 5,494,777 proposes an outward sloping profile in the interface design. U.S. Pat. No. 5,743,346 proposes an interface having an inner surface and an outer chamfer that forms a 5° to 85° angle to the vertical, wherein the inner surface is other than the chamfer. U.S. Pat. No. 5,486,137 also proposes a tool insert having an outer downwardly sloped interface surface. U.S. Pat. No. 5,494,477 proposes a tool insert having an outer downwardly sloping interface. U.S. Pat. No. 5,971,087 also proposes various dual and triple slope interface profiles.
It is still highly desirable to provide a polycrystalline diamond compact having reduced axial, radial, and hoop stresses. It is to such cutters that the present invention is addressed.